1. Industrial Field of the Invention
The present invention relates to a light-weight high-rigidity structure made of thin sheet metal which includes corrugated ribs of a large height formed on bent portions of the structure.
2. Description of Related Art
Recently, saving energy and resources by reducing weights of products has been demanded for solving problems of the global environment. Especially, there has been a strong demand for decreasing the sheet thickness of sheet-metal structures such as shells and covers which have little direct influence on the functions of products. In general, when a sheet-metal box structure such as a shell and a cover is reduced in weight by decreasing its thickness, the resultant decrease in rigidity can be prevented by a complicated sheet-metal structure including more component parts and more fastening portions. However, as an inexpensive method, beads for plane portions or ribs for bent portions or both the beads and the ribs have conventionally been employed.
Design principles of beads and ribs are disclosed in, for example, (1) "Press-Bending Process", written by Akira Hashimoto, published by the Nikkan Kogyo Shinbun, Ltd., (2) "Sheet-Metal Working Methods", written by Kensuke Nozawa, published by the Nikkan Kogyo Shinbun, Ltd., and (3) "Plastic Working", written by Teizo Maeda, published by Seibundo Shinkosha Corporation. According to these literatures, the standard height of beads formed on plane portions is defined 3 to 5 times larger than the sheet thickness. It is known that the rigidity of ribs formed on bent portions is enhanced as the height of the ribs is increased. However, since a difference between peripheral lengths on the larger-radius side and the smaller-radius side in each bent portion is increased, buckling occurs on the smaller-radius side, thereby making the rib formation difficult. Consequently, the standard height of ribs is 2 times larger than the sheet thickness.
However, in order to satisfy the current demand for weight reduction, there is a limit especially in the reinforcement by the conventional ribs on the bent portions. Moreover, when some of the mechanical component elements such as a motor are mounted on a shell or the like, vibrations and noises owing to deterioration of the rigidity caused by decreasing the sheet thickness are inevitably increased.